Receptor tyrosine kinases (RTKs) conduct biochemical signals via lateral dimerization in the plasma membrane. The transmembrane (TM) domains of RTKs play an important role in the dimerization process. A single amino acid mutation in RTK TM domains can cause a defect in cell signaling and result in pathological phenotype. For instance, achondroplasia, the most common form of human dwarfism, is linked to a single amino acid mutation (Gly380-to-Arg) in the TM segment of one RTK, fibroblast growth factor receptor 3 (FGFR3) in more than 97% of all studied cases. Nine years after the discovery of the genetic cause of dwarfism, we have put forward a testable hypothesis for the structural determinants of achondroplasia. We seek to test this hypothesis, and elucidate the structural and thermodynamic consequences of the achondroplasia mutation. We propose to: (1) Determine the dimerization propensities for wild-type and mutant TM domains (TMwt and TMmut) in model systems using Fluorescence Resonance Energy Transfer (FRET), (2) Determine the structures of wild-type and mutant TM dimers using site-directed mutagenesis, FRET, NMR, and molecular modeling, (3) Determine whether Arg380 resides inside the hydrocarbon core of the bilayer, or in its interfacial region, and (4) Assess the ability of mutant TM domain to inhibit unregulated FGFR3 signaling in fibroblasts. The proposed work will (1) enhance our knowledge of the mechanism of dimerization of RTKs, and of cell-signaling across the plasma membrane in general, (2) shed light on the molecular basis of achondroplasia, and (3) pave the way for new treatment options for achondroplasia.